Heart rate modulations in overtraining syndrome

Abstract

Every sports training program includes a component of repetitive overloading, but with an inadequate recovery time, such overloading may produce undesired effects, such as chronic fatigue and a lack of performance improvement. The state of underperformance known as the overtraining syndrome (OTS) is characterized by a number of physiological and psychological symptoms of maladaptation. It may take weeks, months or years to restore proper sports form after the development of OTS. Although overtraining has been studied for decades, the mechanism of overtraining and the tools for the early detection of overtraining are still not defined. In addition to other physiological, biochemical, immunological, psychological and performance markers, heart rate (HR) and its modulations are intensively investigated as a practical and reliable sign of overtraining. In general, resting bradycardia, a decrease in HR during submaximal exercise, an increase in speed of heart rate recovery and increased vagal-related heart rate variability indices are all well accepted markers of improved aerobic fitness. In contrast, changes of these HR measures in the opposite direction are commonly interpreted as indicators of detraining, chronic fatigue, non-functional overreaching or overtraining. However, based on the limited and diverse literature available, these parameters may be used for monitoring training status, optimising training programs and following the accumulation of fatigue, but their role in overtraining detection and assessment has yet to be elucidated. There is a need for well-controlled prospective studies where a longitudinal follow-up of athletes is performed. Because it is unethical to excessively train an athlete to produce OTS, an animal experimental model of OTS may advance our understanding of the 'unexplained underperformance syndrome.'.